Steam generating unit having a superheater and reheater each including a radiant section and a convection section



A 159, 19 I P H KYOCH :ET AL- A 2,948,267

STEAM GENERATINC; UI IIT HAVING A SUPERHEATER AND REHEATER EACH INCLUDING A RADIANT SECTION AND A CONVECTION SECTION Filed May 10, 1955 4 Sheets-Sheet 1 SECONDARY SUPERHEATER I gs mznan. INLET HEADER INTERMEDIATE HEADER FIG.1

SUPERHEATER SIDE-WALLS F'uLL STUD IN VEN TORS PAUL H. KOC H w AM D.STEVENS so By /W ZRRAY WEINER ATTORNEY Aug. 9, 1960 Filed May P. H- KOCH STEAM GENERATING UNIT HAVIN ET A]. G A SUPERHEATER AND REHEATER EACH INCLUDING A RADIANT SECTION AND A CONVECTION SECTION PRIMARY SUPERHEATER INVENTORS PAUL H. KOCH By WI IAM D, STEVENS RAY WEINER ATTORNEY Aug. 9, 1960 P. H. KOCH ETA]. 2,948,267 STEAM GENERATING UNIT HAVING A SUPERHEATER AND REHEATER EACH INCLUDING A RADIANT SECTION AND A cowvscnou ssc'non Filed May 10, 1955 4 Sheets-Sheet 3 v IN VEN TORS ATTORNEY 1960 P. H. KOCH .EI'AI. 2,948,267

STEAM GENERATING mm HAVING A SUPERHEATER AND REHEA'IBR EACH mcwnmc A RADIANT sscnou AND A CONVECTION SECTION Filed May 10, 195 5 4 Sheets-Sheet 4 IN VEN TORS PAUL H. KOCH y IAM D. STEVENS I RRAY WEINER ATTORNEY STEAM GENERATING UNIT A SUPER- HEATER AND REHEATER EACH INCLUDING gEgfigNIANI SECTION AND A CONVECTION Paul H. Koch, Bernardsville, William D. Stevens, Hackensack, and Murray Wiener, Newark, NJ., assignors to The Babcock & Wilcox Company, New York, N.Y., a corporationof New Jersey Filed May 10, 1955, Ser. No. 507,438

7 Claims. (Cl. 122-481) This invention relates to the art of steam generation. It is more particularly concerned with steam generation, superheatingand reheating at high temperatures. and high pressures. The steam pressures are within the range of 1500 to 3000 p.s.i.a., and the steam superheat and reheat temperatures are within .the range of 1000 F. to 1200 F.

The invention is also concerned with the generation, superheating and reheating of high pressure steam in a steam generating unit of high capacity, within the range of 500,000 to 2,000,000 pounds of steam per hour, the unit being fired by a fuel burning means providing combustion gases at a maximum temperature within the range of 2400 F. to 3300 F.

In one of its aspects, the invention is characterized by the passage of high temperature combustion gases to more remote from the fuel burning means, there are widely spaced platens or curtain walls, each consisting of closely arranged radiantly heated steam heating tubes. Preferably, these spaced platens or curtain walls are alternatively secondary steam superheater platens and steam reheater platens.

At the gas outlet of the radiant heat chamber of the unit, the gases enter a convection section which is divided into two parallel gas passes, with one of these gas passes having wholly convection steam reheater surface therein and the other gas pass having wholly steam superheating surface therein. Preferably, both the steam reheater and the secondary superheater are so constructed and arranged that each includes a substantial proportion of the predominantly radiantly heated surface in the radiant heat absorption chamber and a substantial proportion of the convection heated surfaces within the convection section. Such inclusion of both the radiantly heated surfaces and the convection heated surfaces in the reheater and in the superheater promotes the attainment of a predetermined steam temperature over a wide range of rate of vapor generation, because of the opposing inherent tendencies of the predominantly radiantly heated sections and the predominantly convection heated sections, as the rate of vapor generation changes. With this type of arrangement also the ratio of predominantly radiantly heated surface to predominantly convection heated surface in the reheater may be greater than the corresponding ratio in the superheater, so as to compensate for the tendency of the reheater to have a greater loss in steam temperature than the temperature loss of the superheater, as the rate of vapor generation decreases.

The invention also involves a type ofwall construction for the radiant heat zone provided by a secondary furnace chamber, this wall construction involving two rows of heat absorbing tubes within a pressure tight casing con- 2,948,267 Patented Aug. 9, 1960 ICC struction. In this type wall construction the tubes immediately inward of the pressure tight casing are vapor generating wall tubes or risers, and spaced inwardly of and structurally tied to these tubes are the closely arranged tubes of a wall tube steam superheater. The superheater tubes are tied together in wall forming alignment so as to maintain the integrity of the Wall tube superheater, and the wall tube superheater, as an entity,

I is preferably tied to the risers or steam generating tubesinterspersing of the predominantly radiantly heated zones for reheating, relative to the predominantly radiantly heated zones for steam superheating. The steam generating, superheating and reheating unit of ,the invention also involves, in combination with the arrangement above indicated, the disposition of at least some ofthe reheater surface ahead of the superheater surface, with respect to gas flow.

The invention will be concisely as set forth in the of the inventiomits uses and advantages, recourse should be had to the following description which refers to the accompanying drawings in which a preferred embodiment of the invention is illustrated. 35

V of a cyclone furnace fired steam generating, superheating and reheating unit, embodying the invention;

' Fig. 2 is a sectional plan taken on the line 2-2 of arranged within the upper'part of the secondary furnace chamber, this view being taken on the line 3-3 in Fig. 2;

Fig. 4 is a fragmentary horizontal section at a position such as that indicated by the line 4-4 of Fig. 1, and

showing the arrangement of the riser tubes, the exterior gas tight casing arranged along the outer sides of these tubes, and the row of closely spaced steam super-heater tubes which are tied to the larger diameter risers;

Fig. 5 is a fragmentary side elevation of the Fig. 4 structure, on the line 5-5 of Fig. 4;

Fig. 6 is a fragmentary horizontal section of a modified double wall tube wall construction taken on the line 6-6 of Fig. 7;

Fig. 7 is a fragmentary side elevation of the Fig. 6 structure taken on the line 7-7 of Fig. 6;

Fig. 8 is a cut-away perspective view showing a fragmentary part of another embodiment of the double tube row furnace wall construction somewhat similar to the embodiments indicated in Figs. 4 and 6; and

:Fig. 9 is a plan view on a smaller scale of the wall construction illustrated in Fig. 8.

Fig. l of the drawings diagrammatically illustrates a natural circulation steam generating unit having a steam and water drum 10 having therein drum internals, includ- 1 ing steam and Water separating devices 11 such as those claims appended hereto, but for a complete understanding 3 downwardly through large diameter downcomers 12 to a lower drum 14 at the lower part of the unit. Leading upwardly from the drum 14 are a plurality of rows of steam generating furnace wall tubes, :the" intermediate parts .of which delineate the boundaries -of-' the. radiant heat absorption zone formed by the-secondary furnace chamber 16. This chamber is of such large dimensions,

in a high capacity unit, that it-may be. ofa height. of the order of 75 feet, of a Width of the order-of 60 feet between the side walls which connect the front wall 18 and the rear wall 20. These side walls may have a depth of the order'of; 20-30 feet.

The system of steam generating furnace wall tubes of the Fig. 1 unit includes, at-the lower part of the unit, the wall tubes'l2 which are connected at their upper ends --to the intermediate header 24. s From the upper part of this header the wall tubes-26 lead along the upper part-of the wall 20 to the upperwall tube header .28, which isaappropriately connected to the drum 10.

Other -wall tubes 30* lead from. the drum- 14 along the fioor32 of the primary furnace chamber 34, then along the wall components 36-38 which separate the cyclone lower part of each cyclone furnace chamber, and at positions such as those indicated at 48and 50, fused -incombustible such 'as slag, flows through appropriate openings in the wall components 37 and 36 into the primary furnace chamber 34, collects upon the floor 32,

andexits through an opening 33 in that floor.

Parts of the steam generating tubes in the wall components 3638 separating the cyclone furnaces from the 5 primary furnacechamber 34 are included in the constructions providing the cyclone furnace throats 44 and 46. The next superjacent parts 50- of these tubes ex- "tend along the'inclined front wall section '52'to thevertical section of the wall 18 where they continue as the verticalsections 54 to connection with the drum 10, as indicated at 56.

Intermediate the wall tube: sections22 extending along ;the lower part of the furnace-wall 20,. and the wall tube portions between the cyclone. furnaces and the pri- ;-mary.. chamber 34 are other steam generating tubes, the

lower parts of which are indicated at 60. From these lower parts these tubes'continue upwardly in widely spaced screen portions 62 and 64. Above this screen these tubes continue in wall alignment to form the basis for the refractory covered wall 66 dividing theprimary chamber 34 from the lower part of the secondary furnace chamber. Above the upright wall 66 these tubes continue through the inclined parts 68 forming the basis for the refractory lined roof of the primary furnace chamber 34. They then continue along the inclined wall section 52, and between the wall tubes 54 along the vertical front wall 18. They continue upwardly along that wall to such connections with the drum 10 as those indicated at 56. A similar arrangement of steam generating furnace wall tubes is included in the side walls of the furnace chamber, connecting the front wall 18 with the rear wall 20," and such systems of side Wall tubes are arppropriately connected by means of upper headers and lower headers, and other appropriate tubular connections to the drum l and the lower drum 14.

Combustion;.gases with some particles of fusedcombustible suspended therein pass from the cyclonefur- .naces into the primary furnace'charnber at temperatures within the-rangeof 2300 F.. to 3300:F. -;Someiof the suspendedparticles of fused incombustible are deposited collect in the pool of slag 35 along the refi'actory covered slag tap bottom 32. The temperature of the gases is lowered somewhat by the heat absorption of the steam generating tubes forming the Walls of the primary furnace chamber. The gases then pa'ssbetween the screen tubes 62 and 64- into the lower part of the. secondary furnace chamber-16. Thisp'art ofthe secondary furnace chamher, as indicated in F-ig; 1, has Iall of'its walls including upright steam generating tubes having a multiplicity of metallic studs welded thereto for holding a high temperature refractory covering which lines the l'ower-"part'of the' secondary furnace chamber.

' Upwardly fronrthe levelof-the header 24 all of the walls of the secondary furnace 'chamber16, with the exception of the inclined wall '52,.arelined with tangent or closely arranged radiant superheater tubes which will be later referred to. Some'ofthe'setubes are indicated at 70. in Fig. 2 as disposed .along theside wall portion 72,1thef opposite. side wall. portion 7 4 .having similar radiantst1perheater tubes '76.. disposed-therealong. The radiantsuperheater tubesdfiposed along the wall 20' are indicated at '78iin" Fig. 1, :and theradiant superheater .tilb'es' -dispo'sed alongthe frontwall 18 are indicated at in 'tFigfl and FigI'Z.

As the gases -passupwardly through the radiant heat absorption zone immediately above the level of the header -24; the temperature is substantially decreased by the radiant heat transmission therefrom to the radiant superheater wall tubes, and above an. intermediateposition designated as iA-LB. .inlFign 1 .radianttheat is. absorbed from the gases at a much higher rate becauseof the addition to -.the radiant superheater tubes along the walls of the sec- .ondary combustion chambervlfi ofa number of reheater .and secondary superheaterplatens disposed in parallelism withtheside wall portions 72 and'74, and widely spaced .(i.e. 6 feet).across the 60-foot dimension of the secondary furnace chamber. Some of these platens are reheater t. platens as indicated M82, 84, 86, 83 and in Fig. 2, sand the interveningplatens as indicated at 83-, 85, 87 and i 89 are-secondary superheater platens. Each platen prefer- .ably consists of pendent U-tubes'having downflow legs such as thoseindicate d at 90A in Fig. 3 leading downwardly-from. a reheaterheader 96B, and an upflow leg, the main part' of which is in wall tubealignment with the downflow leg. 'The uppermost part of'such an upflow legis. indicatedat 99C in Fig. 3 as discharging into the a header 90D. .The headers 99Band 96D are supported fromappropriatestructural steelwork by suitable hangers which aregenerally. indicated at 90E in Fig; 3. The superheater platen tubes .-are similarly constructed and have similararrangement of headers and tubes for each platen, withtheexception of thesu-perheater secondary furnace wall tube platens immediately adjacentthe-upright steam generating wall-tubes of thesecondary furnace chamber, as-indicatedin the right hand part of Fig. 3. Here the steam generating tubes of the-sidewall' are indicated at 92 .as connected to'a header'94 supported from steelwork by appropriate hangers 96. The U-tubes for theradiant superheater along the secondary furnace chamber wall adjacent the tubes 92-are indicated at'70 Each U-tube may be 1 considered-ashaving an :inlet-leg 70A leading from the inlet Lheader- 76B, and an outlet leg 700 leading to the outlet header; 70D.

aThe roof of the-secondary*furnace chamber 16 also .includes'secondary superheatertubes' 100 constituting con- .tinuationsof the front wall radiant superheater tubes 89, and-serving with these' tubes to conduct steam from the --'secondary'snperheater inlet=header 192-to the intermediate secondary superheater header: 104.

The-walls of thewcyclone furnaces 40 and-42 include vapor generating tubes appopriately connected into the ':natural circulation ofthe unit by means-of suitable headl'ersand: connecting tubes. One of such-headers .is shown 2 at=-106 in. Fig. l. From this header 'risers- 108"leadup- 1 from thegases within the primaryfurnace chamber to 75 wardly and then through "the inclined--wall 52 :in: platen formation with the tubes in closely arranged or con-' tiguous relationship to form the platens 1 -414 as indicated in Fig. 2. These narrower platens are preferably aligned With the reheater platens 82, 84, 86, 88 and 90. The upper ends of these platens are suitably connected by appropriate headers and circulators to the drum 10. The lower ends of the platen tubes may be in communication by suitable connections with the lower drum 14. j r

The combustion gases passing upwardly within the secondary furnace chamber 16 continuepast the level A--B and into the zone of the secondary superheater and reheater platens with a small proportionof the particles of fused incombustible still suspended'therein, but the increased rate of radiant heat transfer from the gases and their suspended slag particles to the tubes of the reheater and secondary superheater platens soon reduces the temperature of the gases to values at which the suspended particles will not stick to and accumulate upon the more closely spaced superheater and reheater tubes. The convection section of the unit is illustrated in Fig. 1 as including a horizontal gas pass leading laterally to the right from the upper end of the secondary furnace chamber 16. As indicated in Fig. 2 this convection section is divided in the direction of fiow of the gases into a reheater gas pass 110 and a superheater gas pass 112 by an upright wall 114A. The heating surfaces within the gas pass 110 are Wholly formed by reheater elements and the convection sections within the gaspass 112 are wholly superheater elements. At the hightemperature or leading part of the gas pass 112 there is the convection superheater section 116 from which the gases continue across the elements of the primary superheater section 118.

From the outlets of the gas passes 110 and 112 the gases turn downwardly for flow through a gas pass 120 'in which there are disposed the convection economizer sections 122 and 124. r 7 Steam fio-ws from the steam space of the drum 10 through a number of lines directed as indicated at 126. Some of these lines have the vertically arranged down- :flowvsections 128 leading to an appropriate inlet header, paired with an adjacent outlet header, both of the headers extending along one side wall of the convection gas pass. Connecting these headers are U-tubes, the downfiow'and upfiow legs of which form wall tubes along a side wall of the .gas pass of the convection section, the arrangeinent of these headers and their connected superheater wall tubes being similar to the arrangement of headers and U-tubes indicated in Fig. 3 for the reheater and superheater platens at the upper portion of the secondary furnace chamber. From the outlet header, paired with the inlet header 130, the steam flowvcontinues through the outlet connections 132 to an intermediate primary superheater header 134.

Others of the steam supply lines 126 leading from the drum 10 have the downflow legs 136 leading to a primary superheater inlet header 138, from which steam flows through a plurality of closely arranged roof tubes 140 along the roof of the gas passes of the convection section. The steam flow through these roof tube sections continues through the upright tube sections 142 at the rear of the convection gas passes; and then through the superheater sections 144 across the entrance to the downflow pass 120. Thence the steam flow continues through the tubular sections 146 and 148 along the floor sections 150 and 152 of the convection gas passes. These steam flow tubes continue downwardly along the rear furnace wall and'inwardly of the riser tubes 26 through the downfiow legs 78 of U-tubes to a level adjacent the header 24, whence the steam flow reverses and continues through Itheupfiow legs 160. Across the gas inlet of the convec tion section these upfiow legs 160 are more widely spaced as indicated at 162 and 164 to form a screen across the as flow'fromthe upper part of the secondary furnace 6 chamber to the convectiongas passes. The steam'flows from these tubes continue to the intermediate header 166 ofthe primary superheater; From this header the steam flows continue through the lines 168 to the mixing header 134. of the primary superheater.

. Described above is one steam flow circuit through tubes alongone side wall ofthe convection gas pass, with the steam fiow...emerging from this circuit into the mixing header 134., ,It is 'to be unders'tood,,however, that there isanotherseries: of steamflow, circuits for the opposite exterior wall of the convection-gas pass, with the steam flows of those circuits also'emerging into'the mixing header 134. Thus there arethree parallel flows of steam in the. primary superheaterfwith the steam from each flow reaching the mixing header 134 before entering the convection section .118 of the vprimary superheater. This convection section consists of a plurality of platens each having multiple looped and series connected U-tubes receiving steam through one of a multiplicity of inlets 170, and ultimately discharging the steam through one of a plurality of outlets 172.into the primary superheater outlet header 174. I

From the header 17.4 the steam flow continues through a plurality of lines 176 through a spray attemperator 178, and then through a corresponding number of lines 180 to the inlet header 102 of the secondary superheater. From this header the steam flows through the inlet legs of continuous U-tubes constituting the roof tubes 100 of the secondary furnace chamber, and the furnace wall tubes 80. From the outlet legs of these U-tubes the steam flows through the secondary superheater intermediate header 104 and thence through connections 184 leading in parallel to the inlet headers for the secondary superheater platens in the upper part of the secondary furnace chamber. One of such headers is indicated at 70B in Fig. 3.

From the outlet headers of the secondary superheater platens or curtain walls in the upper part of the secondary furnace chamber, such as the header 70D'of Fig. 3, the steam flows in parallel through one or more lines 186 to the inlet header 188 of the secondary superheater convection section 116. This secondary superheater section is illustrated as consisting of a plurality of series connect'ed U-tubes conducting steam to the header 190, from which the steam flows to a turbine, or other point of use;

Relative to the secondary superheater there has been described above one flow circuit leading from the inlet header 102 through the roof tubes 100, and then through the furnace front wall secondary superheater tubes and back to the secondary superheater intermediate header 104. There are also two other circuits connected in parallel with this circuit. One of these additional circuits includes appropriate lines leading from the header 102 to such headers as the header 70B which is indicated in Fig. 3 as the inlet header of one of the furnace side wall secondary superheater radiant sections. In this furance wall radiant superheater section the steam flows through the downflow legs of the U-tubes, and then through the adjacent upfiow legs to the outlet sections 70C to the header 70D, and'thence through appropriate lines to the secondary superheater intermediate header 104. The second additional parallel circuit for the secondary superheater between the headers 102 and 104 consists of an arrangement of tubes and headers similar to those indicated in Fig. 3 for the furnace side wall opposite the wall along which the steam generating tubes 92 are disposed. Thus there are three parallel circuits for the secondary furnace chamber wall tube radiant superheaters leading from the inlet'header 102 to the intermediate header 104.

The reheater has appropriate headers and tubular connections through which steam flows from the outlet of one stage of a steam turbine to the inlet headers for the reheater platens at the top of the secondary furnace chamber. One of such inlet headers for the reheater platens is indicated at B in Fig. 3. Steam flows in parallel to all ofthe' headers 90B for the various reheater platens and then downwardly through the downfiow legs of the U-tubes of the platens, then through the upflow legs to theoutlet headers for the reheater platens, one of such headers being indicated at 90D in Fig. 3. Steam flows in parallel from the outlet reheater platen headers 90D through appropriate connections to a spray attemperator and thence through an inlet header for the convection section of the reheater disposed within the reheater gas pass 110. This reheater inlet header is aligned with the mixing header 134 for the primary superheater, and the steam to be reheatedjflows from this header through series connected U-tubes, constituting successive banks of tubes, similar tothe banks of tubes of the primary superheater section 118. Such banks of tubes, or sections of the con! vection section of the reheater, are indicated at 2009-204 in the upper part of Fig. 2. The outlet header for the convection section of the reheater is axially aligned with the header 190 for the convection section 116 of the sec, ondary superheater, as indicated in Fig. 1 of the draw? mgs.

The different wall tube constructions indicated in Figs. 4-9 of the drawings are of types which may be used in a double wall construction for the secondary furnace chamber '16 of the Fig. 1 unit.

Each wall tube arrangement includes a row of relatively large diameter riser tubes disposed in wall alignment exteriorly of a row of smaller diameter radiant superheater tubes also secured in wall alignment. In the arrangement indicated in Figs. 4 and 5 riser tubes 26 are Welded as indicated at 220 and 222, to a buckstay 224-. Such buck= stays are vertically spaced and between the buckstays at successive elevations there are panels 226, outwardly flanged as indicated at 228, and having seal welds 230 uniting the edges of the flanges with the contiguous edges of the flanges 232 of the buckstays. A pressure tight casing is completed by similar seal welds, some of them joining horizontally adjacent panels.

The radiant superheater tubes 78 and 160 are held in wall alignment by rods or pins 240 extending through aligned sleeves 242-444 and held in position by a tack weld to one of these sleeves. Two of the sleeves, such as 2.42 and 244, for example, may be welded to the tube 160 and the intermediate sleeve 243 welded to the adjoining tube 78. Means for tying the tubes together may be used in such a manner that a group of four tubes is thus tied together, whereas the tubes of the next succeeding group of four tubes are tied separately together.

The wall of superheater tubes in the arrangement of Figs. 4 and 5 is held in spaced relationship to the wall of riser tubes 26 by tying means which permit relative movements of the different tube walls. This means consists in T-lugs 250 welded to the tubes 78 and 160 as shown, with the head of the T of each lug embraced by the converging lateral extensions 252 and 254 of the companion lugs 256 and 258 welded to the tube 26 immediately to the rear of the tube 78, to which the member 250 is welded. Such tying means are preferably distributed horizontally and vertically over the face of the radiant superheater wall so that it may be maintained in operative relationship to the wall of riser tubes, while permitting a limited relative movement of the walls formed by the two different tubes.

In the wall construction indicated in Figs. 6 and 7, arrangement of the buckstays and the panels welded thereto is much the same as that described with reference to the construction shown in Figs. 4 and 5. Also, the manner in which the wall superheater tubes are secured in operative relationship to the wall of riser tubes 260 is the, same as that indicated in Figs. 4 and 5.

The wall construction of Figs. 6 and 7, however, includes refractory material 270 preferably installed as a plastic or semiaplastic between the wall of superheater tubes and a Web of reticulated metal 272 secured to and extending between adjacen ubes 2.60. The Fig. 6 and. Fig. 7 arrangements also. inclu e. the rods. 27 disposed i 8 in the grooves formed by the substantially contiguous adjacent tubes and welded to the adjacent tubes as indieated at 275.

In the furnace wall construction of Figs. 8 and 9, the closely arranged tubes 280 of the radiant superheater furnace wall may be secured'in their wall alignment in the manner indicated in Figs. 4 and 6, or the radiant Super-heater tubes may be held in their wall alignment by securement to the superheater buckstay 282. This buckstay is a part of a gasrtight wall including panels such as 2&4 and 286 having circumscribing seal welds securing the panels together at the outer edges of their flanges, and having the edges of some of the panels welded directly to the edge portions of the buckstays, such as 282, all in a manner previously indicated with reference to the type of wall construction indicated in Figs. 4 and 5.

The larger diameter riser tubes 288 of the Fig. 8 con: struction may lead upwardly from a header such as the header 24'of Fig. 1. In this case, the curved casing component 290 extends inwardly and upwardly from the header to junction with a channel expansion joint construction 292, the upper flange 294 of which is disposed adjacent the outer edge portion of the flange 296 of the buckstay 282, and is seal welded thereto to act as a part of the casing construction and to permit relative move: mer t between the superheater tubes 280 and the tubes 288, and the header to which they are secured.

The row of risers 288 of the Fig. 8 and Fig. 9 corn structions is spaced substantially outwardly of the row of radiant superheater tubes 280, and both rows of tubes are held in their operative relationship by guides or connectors 298 and 300 which engages the flanges 302 and 304- of the buckstay 306 to hold the rows of tubes in their operative relation to the buckstay. The connectors 2938 include plates such as 308 secured to the tubes 288 by welding and having upper and lower angular lugs 3'10 and 312 slidably engaging the inner surfaces of the buckstay flanges 302 and 304.

The connector devices 300 for the radiant superheater Wall including the buckstay 282 and the tubes 280 also involve a number of longer plates such as those indicated at 314. The inner ends of these plates are disposed within the channel of the buckstay 282 and are welded thereto as indicated at 316, while the outer ends of the plate members 3-14 have the angular lugs 318 and 320 welded thereto so as to slidably engage the flanges 302 and 304 of the buckstay 306.

Although the invention has been described with par.- ticular reference to the details of a preferred embodiment, it is to be appreciated that the invention is not necessarily limited to all of the details thereof. It is rather to be considered as of a scope commensurate with the scope of the subjoined claims.

What is claimed is:

1. In a steam generating unit having a section including steam generating wall tubes, fuel burning means associated with the steam generating section for developing high temperature combustion gases from which heat is radiantly transmitted to the steam generating wall tubes, steam superheating wall tubes defining a large volume radiant heat absorption zone of a furnace chamber receiving the high temperature gases, a convection section including an arrangement of walls forming a convection gas pass communicating with gas flow from the furnace chamber, a superheater having a convection section disposed within the convection gas pass, the superheater also having a radiant section including widely spaced platens of tubes disposed at a position ahead of the entrance of the convection gas pass, and a reheater having a convection section within a convection gas pass communicating with gas flow from the furnace chamber and also having widely spaced radiant platens of tubes dis? .posed in a zone of high temperature gases ahead of the gas entrance of the convection gas pass and intermedi ate and spaced from said superheater platens, a part of the steam superheating wall tubes being disposed ahead of the reheater platens relative to gas flow.

2. In a steam generating unit, upright steamgenerating tubes disposed along the walls of a vertically elongated secondary furnace chamber, parts of said tubes also defining the boundaries of a primary furnace chamber at the bottom of a unit, a cyclone'furnace burning fuel at temperatures within the range of 2400 F.3300 F. and delivering high temperature combustion gases to the primary furnace chamber for subsequent flow into and upwardly through the secondary furnace chamber, wall means defining a convection gas pass leading from the upper part of the secondary furnace chamber, a superheater having closely arranged furnace wall tubes lining a wall of the secondary furnace chamber, wall means dividing the gas pass into two parallel gas passes, the superheater also having a convection section disposed within one of said parallel gas passes, a reheater having a convection section disposed within the other of said convection gas passes and having a predominantly radiantly heated section including widely spaced platens of closely arranged reheater tubes disposed within the upper part of the secondary furnace chamber and ahead of the gas entrance of the convection gas pass, means providing a flow of steam to be superheated first through said wall tubes and then through said convection section, and means providing a flow of steam to be reheated first through said platens and then through said reheater convection section.

3. A steam generating, superheating and reheating unit comprising walls defining a vertically elongated furnace chamber having fuel burning means providing high temr perature heating gases at its lower end and a convection heating gas pass extending laterally from its upper end, a vertical bafile longitudinally dividing said gas pass into two side-by-side passages, a convection secondary steam superheater positioned in one of said passages adjacent said furnace chamber, a convection primary steam superheater positioned in said passage rearwardly of said secondary superheater, a convection steam reheater in the other passage, radiant steam superheating tubes extending along one of the walls of said furnace chamber, a plurality of groups of steam superheating tubes arranged in widely spaced vertical platens in the upper part of said furnace chamber, a plurality of groups of steam reheating tubes arranged in widely spaced vertical platens in the upper part of said furnace chamber and positioned intermediate and spaced from said superheater platens, means providing a flow of steam to be superheated serially through said furnace chamber wall superheater tubes, convection primary steam superheater, superheater tube platens, and convection secondary superheater, and means providing a flow of steam to be reheated serially through said reheater tube platens and then through said convection steam reheater.

4. A steam generating, superheating and reheating unit ,to be superheated serially through said convection pass roof and floor superheater tubes, convection primary steam superheater, superheater tube platens, and convection secondary superheater, and means providing a flow of steam to be reheated serially through said reheater tube platens and then through said convection steam reheater.

' 5. A steam generating, superheating and reheating unit comprising front, rear and side walls defining a vertically elongated furnace chamber having fuel burning means providing high temperature heating gases at its lower end and a convection heating gas pass extending laterally from its upper end, a vertical baffle longitudinally dividing said gas pass into two side-by-side passages, a convection secondary steam superheater positioned in one of said passages adjacent said furnace chamber, a convection primary steam superheater positioned in said passage rearwardly of said secondary superheater, a convection steam reheater in the other passage, steam superheating tubes extending along the roof and floor of said heating gas pass, radiant steam superheating tubes extending along the walls of said furnace chamber, a plurality of groups comprising front, rear and side walls defining a vertically elongated furnace chamber having fuel burning means providing high temperature heating gases at its lower end and a convection heating gas pass extending later: ally from its upper end, a vertical baflle longitudinally dividing said gas pass into two side-by-side passages, a convection secondary steam superheater positioned in one of said passages adjacent said furnace chamber, a convection primary steam superheater positioned in said passage rearwardly of said secondary superheater, a conof steam superheating tubes arranged in widely spaced vertical platens in the upper part of said furnace chamber, a plurality of groups of steam reheating tubes arranged in widely spaced vertical platens in the upper part of said furnace chamber and positioned intermediate and spaced from said superheater platens, means providing a flow of steam to be superheated serially through said convection pass roof and floor superheater tubes, furnace chamber rear wall superheater tubes, convection primary steam superheater, front and side wall superheater tubes, superheater tube platens, and convection secondary superheater, and means providing a flow of steam to be reheated serially through said reheater tube platens and then through said convection steam reheater.

6. In a steam generating unit, upright walls defining a vertically elongated furnace chamber and including steam generating tubes, means normally providing high temperature gases for the unit, means providing a convection section including a convection gas pass receiving gases from the upper part of the furnace chamber, a steam superheater including a convection section disposed within the convection gas pass and also including widely spaced pendent platens each having closely arranged superheater tubes disposed Within the upper part of the furnace chamber, and a reheater including a convection section within the convection gas pass and a plurality of widely spaced pendent platens of closely arranged reheater tubes disposed in the upper part of the furnace chamber, the reheater platens alternating with and spaced from the platens of superheater tubes across the furnace chamber, each of said platens being formed by a series of single looped U-tubes having the legs of each tube substantially in contact throughout their length and aligned in the vertical plane of the corresponding platen, and a pair of steam inlet and outlet headers extending along the upper end of each platen and each connected to the corresponding leg of said U-tubes.

7. In a steam generating unit, walls defining a radiant heat absorption zone, said walls including steam generat ing tubes, furnace fuel burning means providing high temperature combustion gases for flow through the radiant heat zone, a steam heater including a convection section disposed within the gas flow beyond the radiant heat zone and having a radiant section including a continuous row of closely arranged tubes extending along a wall of the radiant heat zone and disposed inwardly of and in a vertical plane parallel to a row of steam generating tubes of that zone, tying means extending between and holding said steam heating tubes in inwardly spaced relation to said last named generating tubes and permitting relative vertical movement therebetween, and a gas-tight sheet metal casing fixedly and closely related to the row of steam generating tubes and disposed on the side of the 

